Homoconjugation

1 Extended through space interactions homoconjugation and homohyperconjugation 

The potential for cation stabilization provided by delocalization through space is significant. For example, a properly positioned alkene provides dramatic acceleration (10 ) of ionization of anti C-X bonds with perfect stereospecificity of substitution.  

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In the Hiickel treatment, atomic orbitals on nonadjacent atoms are assumed to have no interacticai. They are neither bonding nor antibonding. The concept of homoconjugation suggests that such orbitals may interact, especially in rigid structures which direct orbitals toward one another. Consider, for example, bicyclo[2.2.1]hepta-2,S-diene ... 

Conjugated, cross-conjugated, and homoconjugated fluoroalkenes react with halogens to yield predominantly 1,4-adducts Results from the reactions of a series of conjugated fluoroalkenes with elemental halogens are summarized in Table 4 In nearly all cases, the fraMs-l,4-addition products are formed exclusively [Il ... 

Bromination of the homoconjugated 5-(difluoromethylene)-6 6-difluoro-2-norbornene yields different product mixtures under ionic and free-radical conditions 1771 (equation 6)... 

Double-bond migration often passes unnoticed, for unless tracers are employed, there may be no direct evidence remaining that migration has occurred. Nonetheless, the fact that it does occur can have a number of important consequences. Selective removal of cis homoconjugated dienes and trienes in natural oils, used to make edible hydrogenated fats, depends mainly on prior isomerization of multiple unsaturation into conjugation under hydrogenation conditions (J9). 

Employing a C2 symmetry in the case of the thiirene 1-dioxide and remembering that the spiro-operator that mixes the fragment orbitals gives nonzero matrix elements only if these orbitals are symmetric to the C2 operation53, the net result is stabilizing. On the other hand, thiirene 1-oxide suffers a homoconjugative destabilization. 

Finally, an electrochemical reduction of bis-a-bromobenzyl sulfone to stilbene195 and a spectacular, so-called bis-homoconjugative, version of the Ramberg-Backlund reaction, which converts the a-chlorosulfone 100 into the bridged cyclooctatriene derivative 101 (equation 63)196-197, have also been published. 

In contrast to the series of hydrocarbons, heterocyclic analogues, even of cyclonona-l,4,7-triyne 2 (u = 3), are known. This is because heteroatom linkers more easily adopt smaller bonding angles and thereby provide some relief of overall angle strain. Since heteroatoms have different electronic properties to carbon atoms, the perceived homoconjugative and homoaromatic effects might be expressed more pronouncedly in heterocyclic [n]pericyclines. 

Structural Features of [n]Pericyclines and the Quest for Homoconjugation as well as Homoaromaticity... 

Fig. 5. [5]Pericyclines with conceived cyclic homoconjugation and neutral homoaromaticity...

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